Thermocouple with a diffused chromium casing

ABSTRACT

A thermocouple has a diffused casing of chromium to prevent oxidation and reaction with a heating fuel.

United States Patent Tyler June 24, 1975 THERMOCOUPLE WITH A DIFFUSED [56] References Cited CHROMIUM CASING UNITED STATES PATENTS {75] Inventor: Hugh J. Tyler, Santa Ana, Calif. l,65l,750 12/1927 Brophy l. 136/233 2,340,899 2/l944 {73] Asslgnee: Robertshaw Controls Company, 2571 700 lo/lgsl Richmond, Va. 3,085,125 4/1963 Hill 136/233 [22] Filed: May 29, 1973 P E v r R P d rzmary xammer er in en egrass [2 Appl' 364588 Attorney, Agent, or Firm-Anthony A. OBrien [52] US. Cl 136/233; 73/359 [57] ABSTRACT [51] Int. Cl H0lv l/04 A thermocou I ple has a diffused casing of chromium to [58] held Search 136/220 prevent oxidation and reaction with a heating fuel.

8 Claims, 2 Drawing Figures THERMOCOUPLE WITH A DIFFUSED CHROMIUM CASING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to thermocouples for sensing elevated temperatures, and in particular, to thermocouples capable of withstanding flames.

2. Description of the Prior Art Many prior art flame sensing systems for gas fired appliances have thermocouple devices immersed in a continuous burning pilot flame. Nickel alloy thermocouple elements are generally the most suitable elements for use in thermocouple devices capable of withstanding flame temperatures for years; however, such nickel alloy elements must be provided with spaced protective sheaths to make the device suitable for immersion in a flame and to avoid large carbon deposits on the elements caused by the catalytic property of nickel to crack gas. As exemplified by US. Pat. Nos. 1,581,380, 1,937,199, 2,012,112, 2,034,539, 2,705,746, 2,705,747 and 3,001,126, some of the prior art sheaths are made from alloys, such as stainless steels, containing a substantial portion of chromium, or are made of copper which is coated with a chromium layer. While spaced protective sheaths protect the thermocouple against oxidation or the formation of unsuitable deposits thereon, the sheath slows down the time of response to the thermocouple; thus where a fast response is required such sheathed thermocouples are not generally acceptable.

A number of prior art thermocouple elements have one or more elements made from alloys containing significant portions of chromium, as exemplified by US. Pat. Nos. 971,767, 1,169,611, 1,315,205, 1,393,375, 2,691,690 and 3,411,956; however, such chromium containing alloys generally contain substantial portions of nickel and are subject to deposition of carbonaceous products when immersed in a gaseous fuel flame or are otherwise unsuitable for sensing flames.

SUMMARY OF THE INVENTION The invention is summarized in that a thermocouple includes an electropositive element, an electronegative element joined to the electropositive element to form a thermocouple junction, and a casing of chromium diffused in the exposed surfaces of the joined elements.

An object of the invention is to provide a thermocouple which is resistant to oxidation or deteriation at high temperatures.

Another object is the elimination of carbonaceous deposits on a thermocouple exposed to a flame.

It is also an object of the invention to provide a thermocouple construction which is both flame resistant and fast in response.

An advantage of the invention is that a diffused chromium casing offers improved protection without shorting out a thermocouple appreciably.

Additional features of the invention are the provision of nickel alloy elements, the provision of a casing having essentially pure chromium on the surfaces of the elements, and the provision of diffused chromium to a depth of at least 0.127 millimeters.

Other objects, advantages and features of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view ofa thermopile having a plurality of serial thermocouples in accordance with the invention.

FIG. 2 is a cross-section view of one of the thermocouple junctions of FIG. 1 along line 22.

DESCRIPTION OF THE PREFERRED EMBODIMENT As illustrated in FIG. 1, the invention is embodied in a thermopile having electropositive elements 10 and electronegative elements 12 joined together to form thermocouple junctions l4 and 16. The junctions 14 are hot junctions and the junctions 16 are cold junctions or vice versa.

The electropositive element 10 and the electronegative element 12 are made from any respective suitable metal or alloy which can be employed at high temperatures. Examples of suitable high temperature electropositive alloys are nickel alloys containing chromium and other minor constituents; one such electropositive alloy being CHROMEL P from Hoskins Manufacturing Company, Detroit, Michigan. Examples of suitable high temperature electronegative alloys are nickel alloys containing copper and other minor constituents; one such electronegative alloy being COPEL XM from Hoskins Manufacturing Company.

As illustrated in FIG. 2 by stippling, the exposed surfaces of the thermopile have an overlapping gradient or casing 20 ofchromium diffused therein. The chromium content of the casing 20 varies from essentially or almost pure chromium at the surface (the first molecular layer) of the thermopile with diffused chromium decreasing to almost zero towards the interior of the elements l0 and 12. Casings of chromium diffused to at least a depth of 0.127 millimeters (0.005 inches) are generally satisfactory, and chromium can be diffused to the center of thermocouple devices without completely destroying the thermocouple properties.

The casing 20 is formed by a conventional diffusion process, such as gas diffusion or chromium vapor diffusion at elevated temperatures. Examples of suitable diffusion processes are described in US. Pat. Nos. 2,874,070 and 3,127,283. One suitable chromium diffusion process is known in the trade as Alphatizing.

In use, the hot junctions 14 are directly immersed in a flame of gaseous fuel while the cold junctions l616 are maintained spaced from the flame and at a lower temperature. A voltage or potential generated by the thermopile is sensed to detect the presence of the flame. The thermopile with the casing 20 of chromium is not subject to deteriation by high temperature oxidation and is substantially immune to deposits of carbonaceous products from burning fuel. The thermopile changes potential immediately with changes in temperature at the hot junctions 14.

The chromium casing 20 has been found to give protection without appreciably shorting out the thermocouple junctions 14 and 16, providing a thermocouple construction which is both flame resistant and fast in response.

Since many variations, modifications and changes in detail can be made in the present embodiment, it is intended that all matters contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A thermocouple adapted for sensing a flame comprising an electropositive element,

an electronegative element joined to the electropositive element to form a thermocouple junction,

at least one of the elements containing a metal which causes formation of carbon desposits on the one element when exposed to burning fuel, and

a casing of chromium diffused in the exposed surfaces of the joined elements,

said casing decreasing in chromium content from essentially pure chromium at the surface of the elements to a substantially less chromium content within the elements.

2. A thermocouple as claimed in claim 1 wherein the one element is an alloy containing a substantial portion of nickel.

3. A thermocouple as claimed in claim 1 wherein both of the elements are alloys containing substantial portions of nickel and the alloy of the one element contains a substantial portion of copper.

4. A thermocouple as claimed in claim 1 wherein the chromium is diffused to a depth of at least 0.127 millimeters.

5. A thermocouple as claimed in claim 2 wherein the chromium is diffused to a depth of at least 0. I27 millimeters.

6. A thermocouple adapted for sensing a flame comprising an electropositive element,

an electronegative element joined to the electropositive element to form a thermocouple junction,

one of said elements being made of an alloy containing a substantial portion of nickel and a substantial portion of copper, and a casing of chromium diffused in the exposed surfaces of the joined elements to a depth of at least 0.127 millimeters,

said casing decreasing in chromium content from essentially pure chromium at the surface of the elements to a substantially less chromium content within the elements.

7. A thermocouple as claimed in claim 6 wherein both the elements contain a substantial portion of nickel.

8. A thermopile adapted for sensing a flame comprisa plurality of electropositive elements of a first alloy containing a substantial portion of nickel, a plurality of electronegative elements of a second alloy containing a substantial portion of nickel,

said electropositive elements being joined in alternate interposed relationship with said electronegative elements to form a plurality of serial thermocouple junctions. and

a casing of chromium diffused in the exposed surfaces of the joined elements to provide oxidation resistance and immunity to carbonaceous deposits, said casing decreasing in chromium content from essentially pure chromium at the surface of the elements to a substantially less chromium content within the elements. 

1. A THERMOCOUPLE ADAPTED FOR SENSING A FLAME COMPRISING AN ELECTROPOSITIVE ELEMENT, AN ELECTRONEGATIVE ELEMENT JOINED TO THE ELECTROPOSITIVE ELEMENT TO FORM A THERMOCOUPLE JUNCTION, AT LEAST ONE OF THE ELEMENTS CONTAINING A METAL WHICH CAUSES FORMATION OF CARBON DEPOSITS ON THE ONE ELEMENT WHEN EXPOSED TO BURNING FUEL, AND A CASING OF CHROMIUM DIFFUSED IN THE EXPOSED SURFACES OF THE JOINED ELEMENTS, SAID CASING DECREASING IN CHROMIUM CONTENT FROM ESSENTIALLY PURE CHROMIUM AT THE SURFACE OF THE ELEMENTS TO A SUBSTANTIALLY LESS CHROMIUM CONTENT WITHIN THE ELEMENTS.
 2. A thermocouple as claimed in claim 1 wherein the one element is an alloy containing a substantial portion of nickel.
 3. A thermocouple as claimed in claim 1 wherein both of the elements are alloys containing substantial portions of nickel and the alloy of the one element contains a substantial portion of copper.
 4. A thermocOuple as claimed in claim 1 wherein the chromium is diffused to a depth of at least 0.127 millimeters.
 5. A thermocouple as claimed in claim 2 wherein the chromium is diffused to a depth of at least 0.127 millimeters.
 6. A thermocouple adapted for sensing a flame comprising an electropositive element, an electronegative element joined to the electropositive element to form a thermocouple junction, one of said elements being made of an alloy containing a substantial portion of nickel and a substantial portion of copper, and a casing of chromium diffused in the exposed surfaces of the joined elements to a depth of at least 0.127 millimeters, said casing decreasing in chromium content from essentially pure chromium at the surface of the elements to a substantially less chromium content within the elements.
 7. A thermocouple as claimed in claim 6 wherein both the elements contain a substantial portion of nickel.
 8. A thermopile adapted for sensing a flame comprising a plurality of electropositive elements of a first alloy containing a substantial portion of nickel, a plurality of electronegative elements of a second alloy containing a substantial portion of nickel, said electropositive elements being joined in alternate interposed relationship with said electronegative elements to form a plurality of serial thermocouple junctions, and a casing of chromium diffused in the exposed surfaces of the joined elements to provide oxidation resistance and immunity to carbonaceous deposits, said casing decreasing in chromium content from essentially pure chromium at the surface of the elements to a substantially less chromium content within the elements. 